Soil and Water Conservationfor Productivity and Environmental ProtectionFourth Edition

Frederick R. Troeh Professor Emeritus of Agronomy Iowa State University J. Arthur Hobbs Professor Emeritus of Agronomy Kansas State University Roy L. Donahue Late Professor Emeritus of Soil Science Michigan State University

Prentice Hall Upper Saddle River, New Jersey 07458

Library of Congress Cataloging in Publication Data Troeh, Frederick R. Soil and water conservation for productivity and environmental protection / Frederick R. Troeh, J. Arthur Hobbs, Roy L. Donahue.4th ed. p. cm. Includes bibliographical references. ISBN 0-13-096807-2 (alk. paper) 1. Soil conservation. 2. Water conservation. I. Hobbs, J. Arthur (James Arthur) II. Donahue, Roy Luther III. Title. S623.T76 2004 631.45dc21

2002192987

Editor in Chief: Stephen Helba Executive Editor: Debbie Yarnell Editorial Assistant: Jonathan Tenthoff Managing Editor: Mary Carnis Production Editor: Emily Bush, Carlisle Publishers Services Production Liaison: Janice Stangel Director of Manufacturing and Production: Bruce Johnson Manufacturing Buyer: Cathleen Petersen Creative Director: Cheryl Asherman Cover Design Coordinator: Miguel Ortiz Marketing Manager: Jimmy Stephens Cover Photo: Numerous terraces help to control erosion on this cropland near Miniato in northern Italy. Courtesy of F. R. Troeh Copyright 2004, 1999, 1991, 1980 by Pearson Education, Inc., Upper Saddle River, New Jersey 07458. Pearson Prentice Hall. All rights reserved. Printed in the United States of America. This publication is protected by Copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department. Pearson Prentice Hall is a trademark of Pearson Education, Inc. Pearson is a registered trademark of Pearson plc Prentice Hall is a registered trademark of Pearson Education, Inc. Pearson Education LTD. Pearson Education Australia PTY, Limited Pearson Education Singapore, Pte. Ltd. Pearson Education North Asia Ltd. Pearson Education Canada, Ltd. Pearson Educacin de Mexico, S.A. de C.V. Pearson EducationJapan Pearson Education Malaysia, Pte. Ltd.

10 9 8 7 6 5 4 3 2 1 ISBN 0-13-096807-2

ContentsPreface 1 Conserving Soil and Water11 12 13 14 15 16 17 Needs Increasing with Time, 2 Erosion Problems, 3 Obstacles to Conservation, 7 Conservation Viewpoint, 9 Conservation Techniques, 10 Choosing Conservation Practices, 14 Caring for the Land, 16 Summary, 17 Questions, 18 References, 18

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Soil Erosion and Civilization21 22 23 24 Origin of Agriculture, 20 Erosion in the Cradle of Civilization, 21 Erosion in Mediterranean Lands, 22 Erosion in Europe, 25

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25 26 27 28 29 210

Erosion in Russia and Associated Nations, 31 Erosion in Asia, 32 Erosion in the Americas, 35 Erosion in Australia, 39 Erosion in Africa, 39 Expanding Interest in Conservation, 42 Summary, 43 Questions, 43 References, 44

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Geologic Erosion and Sedimentation31 32 33 34 35 36 37 38 The Great Leveler, 46 Rock Types, 46

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Processes that Elevate Land, 48 Landscape Development, 51 Sedimentary Landforms, 61 Mass Movement Deposits, 67 Glacial Landscapes, 68 Rate of Geologic Erosion, 68 Summary, 69 Questions, 71 References, 71

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Water Erosion and Sedimentation41 42 43 Types of Water Erosion, 73 Erosion Damage, 76 Agents Active in Water Erosion, 81

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44 45 46 47 48 49

Soil Properties and Soil Erodibility, 90 Vegetation and Water Erosion, 93 Traffic and Water Erosion, 94 Water Erosion and Pollution, 94 Water Erosion and Sedimentation, 95 Principles of Water-Erosion Control, 96 Summary, 97 Questions, 97 References, 98

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Wind Erosion and Deposition51 52 53 54 55 56 57 58 Types of Soil Movement, 101 Erosion Damage, 101 Erosiveness of Surface Wind, 107 Initiation of Soil Movement by Wind, 110 Wind and the Erosion Process, 112 Factors Affecting Wind Erosion, 114 Windbreaks and Shelterbelts, 119 Principles of Wind-Erosion Control, 125 Summary, 125 Questions, 126 References, 126

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Predicting Soil Loss61 62 63 Tolerable Soil Loss, 129 The Universal Soil Loss Equation (USLE), 131 Revised Universal Soil Loss Equation (RUSLE), 139

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64 65 66 67 68

Computer-Based Soil Loss Prediction Models, 149 The Wind-Erosion Prediction Equation (WEQ), 154 Expanded Use of the Wind-Erosion Prediction Equation, 168 Revised Wind Erosion Equation (RWEQ), 171 The Wind Erosion Prediction System (WEPS), 172 Summary, 173 Questions, 174 References, 174

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Soil Surveys as a Basis for Land Use Planning71 72 73 74 Soil Surveys, 180 Soil Map Unit Interpretations, 186 Managing Land, 191 Land Use Planning, 196 Summary, 202 Questions, 203 References, 204

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Cropping Systems81 82 83 84 85 86 Plant Cover, 206 Managing Monocultures, 213 Crop Rotations, 217 Multiple Cropping, 223 Strip Cropping, 226 Evaluating Cropping Systems, 232 Summary, 237 Questions, 239 References, 239

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Tillage Practices for Conservation91 92 93 94 95 96 97 98 Objectives and Effects of Tillage, 242 Types of Tillage Implements, 245 Tillage, Crop Residue, and Soil Properties, 254 Flat Versus Ridged Tillage and Planting, 255 Conservation Tillage, 256 Deep Tillage, 267 Contour Cultivation, 270 Emergency Wind-Erosion Control, 275 Summary, 277 Questions, 278 References, 279

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10 Conservation Structures101 102 103 104 105 106 107 Terraces and Diversions, 282 Gully-Control Structures, 295 Road Ditches and Culverts, 300 Earthen Dams, 301 Streambank Protection, 305 Flood Control, 307 Wind Erosion-Control Structures, 311 Summary, 312 Questions, 313 References, 314

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Vegetating Drastically Disturbed Areas111 112 Construction Sites, 316 Mined Areas and Mine Spoils, 321

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113 114 115

Areas of High Erosion Hazard, 328 Sand Dunes, 341 Disturbed Alpine and Arctic Sites, 345 Summary, 346 Questions, 347 References, 348

12 Pastureland, Rangeland, and Forestland Management121 122 123 124 Pastureland, Rangeland, and Forestland, 350 Pastureland Management, 352 Rangeland Management, 356 Forestland Management, 362 Summary, 372 Questions, 374 References, 374

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13 Water Conservation131 132 133 134 135 136 137 138 139 The Water Cycle, 377 What Is Drought? 380 Combating Drought 382 What Happens to Rainfall? 382 Decreasing Runoff Losses, 385 Reducing Evaporation Losses, 394 Reducing Deep Percolation Losses, 397 Storing Water in Soil, 398 Efficient Use of Stored Soil Water, 402 Summary, 406

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Questions, 407 References, 408

14 Soil Drainage141 142 143 144 145 146 147 148 149 Value of Undrained Wetlands, 412 Occurrence of Wetlands, 413 Characteristics of Wet Soils, 416 Limitations Resulting from Wetness, 420 Water Removed by Drainage, 423 Surface Versus Subsurface Drainage, 426 Methods of Removing Water, 426 Random, Regular, and Interceptor Drains, 436 Design Factors for Drainage Systems, 436 Summary, 444 Questions, 444 References, 445

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15 Irrigation and Reclamation151 152 153 154 155 156 157 158 Effects of Irrigation, 448 Selecting Land for Irrigation, 452 Water for Irrigation, 453 Distributing Water, 462 Irrigation Methods, 467 Irrigation Frequency, 477 Land Reclamation, 478 Conservation Irrigation, 483

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Summary, 486 Questions, 486 References, 487

16 Soil Pollution161 162 163 164 165 166 167 168 Concern About Pollution, 490 Sources of Pollutants, 492 People-Related Wastes, 492 Industrial Wastes, 496 Agricultural Wastes, 498 Aerosols, 506

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Polluted Soil, 507 Hazardous Sites and Soil Remediation, 515 Summary, 518 Questions, 519 References, 519

17 Water Quality and Pollution171 172 173 174 The Earths Water Supply, 523 Water Pollutants, 527 Acidification of Water, 542 Groundwater Contamination, 544 Summary, 546 Questions, 547 References, 547

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18 Economics of Soil and Water Conservation181 182 Benefits from Soil and Water Conservation, 552 Costs of Conservation Practices, 561

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Paying for Soil and Water Conservation, 567 Conservation Incentives, 570 Summary, 570 Questions, 571 References, 572

19 Soil and Water Conservation Agencies in the United States191 192 193 194 195 196 197 198 Early Work on Soil and Water Conservation, 575 Natural Resources Conservation Service (NRCS), 579 Conservation Districts, 587 Farm Service Agency (FSA), 588 Research, Education, and Economics, 590 Universities and Colleges, 590 U.S. Forest Service and State Forestry Agencies, 591 Other Federal Conservation Agencies, 593 Summary, 594 Questions, 595 References, 595

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20 Soil and Water Conservation Around the World201 202 203 204 205 206 Worldwide Needs for Soil and Water Conservation, 597 Transfer of Conservation Technology, 599 Food and Agriculture Organization, 600 Shifting Cultivation and Conservation, 600 Soil and Water Conservation in Selected Areas, 605 Only a Sampling, 617 Summary, 618 Questions, 619 References, 619

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Appendix AConservation Factors Appendix BCommon and Scientific Names of Plants Mentioned in the Text Index

623 627 641

Preface

Soil and water have always been vital for sustaining life, and these resources are becoming more limiting and crucial as population increases. The importance of conserving soil productivity and protecting the quality of both soil and water is becoming clear to more people than ever before. Declining productivity and increasing pollution could spell disaster for all residents of the Earth. The soil and water resources of the planet are finite and are already under intensive use and misuse. Environmental degradation is becoming painfully evident, and increasing numbers of people are demanding that steps be taken to not only reduce the amount of current degradation but also to amend some of the previous damage. Soil and water conservation deals with the wise use of these important resources. Wise use requires knowledge, understanding, and value judgments. The hazards posed by erosion, sedimentation, and pollution, and the techniques needed to conserve soil and maintain environmental quality are all treated in this book. Situations and examples are drawn from many places to constitute a cross-section of the soils, climates, and cultures of the world. The scope includes agricultural, engineering, mining, and other uses of land. Soil and water are recognized as essentials for everyones life. This fourth edition continues the use of foot-pound-second units as the principal units of measurement. Metric units are usually included in parentheses and are presented as the principal or only units where they are the units generally used in the United States. The fourth edition has been updated throughout with many citations to the literature published since the third edition was printed. Significant new material has been added, and certain sections have been expanded. The trend toward computerizing the soil-loss equations is emphasized in Chapter 6. The rapidly growing use of no-till cropping is recognized with an expanded treatment in Chapter 9. The former chapters Vegetating Mining and Construction Sites and Vegetating Other Areas of High Erosion Hazard have been combined in one chapter titled Vegetating Drastically Disturbed Areas. This and several smaller changes helped to consolidate similar topics and make the material flow more smoothly. The increased emphasis on water conservation initiated in the third edition is continued in this edition. Much of this book can be read and understood by anyone with a good general education. Some parts, however, necessarily assume an acquaintance with basic soil properties such as texture, structure, water-holding capacity, and cation exchange capacity. These

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topics are covered in any introductory soil science textbook and one of these should be consulted if the reader lacks this background. The system of soil taxonomy used in the United States is followed in this book. An explanation of that system can also be found in modern introductory soils textbooks. The broad collective background of the authors in soil science and soil conservation in the United States and abroad has been reflected in each edition and carries forward into this new edition. Much credit goes to Dr. Roy Donahue for having originated this project, enlisted his co-authors, and contributed enthusiastically to all three previous editions in spite of his advancing age. However, his death in 1999 made it necessary to change the handling of revisions for this edition. Suggestions were obtained from several users of the present text, and Dr. Troeh accepted the responsibility of incorporating these suggestions along with new material from the literature into the text. Dr. Hobbs contributed by reviewing all of the material and making valuable suggestions. The authors experience has been supplemented by extensive use of excellent libraries to locate appropriate literature including journals, books, and publications from government agencies. Many colleagues have also contributed valuable suggestions and some have thoughtfully reviewed the manuscript of one or more chapters dealing with subject matter in which they were especially well qualified. The helpful assistance of the following persons is gratefully acknowledged: Paul L. Brown, ARS-USDA, Northern Plains Soil and Water Research Center, Bozeman, Montana (deceased) Lee Burras, Associate Professor of Agronomy, Iowa State University J. Brian Carter, Oklahoma State University Julian P. Donahue, Assistant Curator, Entomology, Natural History Museum, Los Angeles County, California George R. Foster, ARS-USDA, National Sedimentation Laboratory, Oxford, Mississippi (retired) Paula M. Gale, Associate Professor, Plant and Soil Science, University of Tennessee at Martin Harold R. Godown, NRCS-USDA (retired) Robert Gustafson, Botanist, Natural History Museum, Los Angeles County, California Lawrence J. Hagen, ARS-USDA, Northern Plains Area Wind Erosion Research Unit, Kansas State University, Manhattan, Kansas Walter E. Jeske, NRCS-USDA, Washington, D.C. John M. Laflen, Laboratory Director and Research Leader, USDAARS National Soil Erosion Research Laboratory, Purdue University, Lafayette, Indiana Rattan Lal, Agronomy Department, Ohio State University John Malcolm, USAID, Washington, D.C. Gerald A. Miller, Associate Dean of Agriculture and Professor of Agronomy, Iowa State University John A. Miranowsky, Professor of Economics, Iowa State University

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Basil Moussouros, former Minister for Agriculture, Government of Greece Kenneth R. Olson, Professor of Crop Sciences, University of Illinois Gerald W. Olson, former Professor of Soil Science, Cornell University (deceased) G. Stuart Pettygrove, Department of Land, Air, and Water Resources, University of California at Davis Durga D. Poudel, Assistant Professor of Soil Science, University of Louisiana at Lafayette Kenneth G. Renard, ARS-USDA, Southwest Watershed Research Center, Tucson, Arizona (retired) E. L. Skidmore, ARS-USDA, Northern Plains Area Wind Erosion Research Unit, Kansas State University, Manhattan, Kansas Barbara M. Stewart, NRCS-USDA, Des Moines, Iowa Gene Taylor, formerly U. S. Congress from 7th District of Missouri (deceased) Glen A. Weesies, NRCS-USDA, National Soil Erosion Research Laboratory, Purdue University D. Keith Whigham, Professor of Agronomy, Iowa State University C. M. Woodruff, Professor Emeritus, Department of Agronomy, University of Missouri (retired)

Frederick R. Troeh Ames, Iowa J. Arthur Hobbs Winnipeg, Canada

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1CONSERVING SOIL AND WATERSoil and water are vital resources for the production of food, fiber, and other necessities of life. Food and fiber are renewable resourcesa fresh crop can be grown to replace what is consumed. The soil that produces these renewable resources is essentially nonrenewable. Water can recycle, but its supply is limited, and it is frequently the limiting factor for crop production. Strong reactions occur when there are shortages of food products or other consumer items. Prices of coffee and sugar, for example, have increased dramatically when a significant part of the worlds crop was damaged, often by unfavorable weather, temporarily decreasing the supply. Such situations arise suddenly and require adjustments in the lives of many people. The more gradual changes resulting from persistent processes such as soil erosion may escape attention despite their fundamental importance. The long-term loss of productivity caused by soil erosion should be of greater concern than temporary shortages. The purpose of soil conservation is not merely to preserve the soil but to maintain its productive capacity while using it. Soil covered with concrete is preserved, but its ability to produce crops is lost in the process. Intensive cropping uses the soil but often causes erosion on sloping land. Land needs to be managed for long-term usefulness as well as for current needs; that is, its use should be sustainable. Scarred landscapes, as shown in Figure 11, tell a sad story of waste and ruin where long-term principles have been sacrificed for short-term gain. Soil erosion is often more detrimental than might be supposed from the amount of soil lost. The sorting action of either water or wind removes a high proportion of the clay and humus from the soil and leaves the coarse sand, gravel, and stones behind. Most of the soil fertility is associated with those tiny particles of clay and humus. These components are also important in microbial activity, soil structure, permeability, and water storage. Thus, an eroded soil is degraded chemically, physically, and biologically. Degradation of soil and water resources is a worldwide problem that takes many forms (Napier et al., 2000). It is especially severe in developing countries where people are struggling to eke out an existence and are more concerned with survival than with conservation. Each situation is different and calls for its own distinct solution.1

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Conserving Soil and Water

Chap. 1

Figure 11 The amount of soil eroded by gullies eating their way into a landscape is spectacular but is often exceeded by sheet erosion around the gullies. (Courtesy USDA Natural Resources Conservation Service.)

11 NEEDS INCREASING WITH TIME The demand for plant and animal products increases with time as population increases and standards of living are raised. People now consume more food than do all other land animals combined (Deevey, 1960). Their needs place an increasing load on soil productivity a load that can severely strain the ecosystem (Whlke et al., 1988). Plants can be grown without soil by hydroponics and by sand or gravel culture, but the expense is high and the scale is small. Even seafood is used on a much smaller scale than are soil products. Until recent decades, production increases came mostly by using more land. New frontiers were opened, forests were cut, prairies were plowed, and deserts were irrigated. It was suggested that one hectare (2.5 acres) of cropland per person was needed to maintain a satisfactory standard of living. A continually expanding land base maintained approximately that much area for a long time. Of course, the best land was chosen first, so the average suitability of the land declined even while the area per person was maintained. The one-hectare-per-person rule is no longer supported. Most countries now have more people than hectares of cropland; the world average is declining and will soon be down to 0.1 ha per person (Lal, 1999). Production depends on soil, crop, climate, and management as well as land area. One hectare per person may not be adequate in some places, but it is enough to support ten or twenty people in other places. In recent decades, the land base has been relatively constant. Most of the good cropland is already in use. Irrigation has been increasing and may continue to increase, but much

Sec. 12

Erosion Problems

3

of the newly irrigated land comes from previously rainfed cropland. The small areas of new cropland being added each year are offset by new roads and buildings on former cropland. Ryabchikov (1976) estimates that people already are using 56% of Earths land surface, 15% of it intensively. Much of the rest is covered by glaciers, bare rock, steep slopes, desert, or other conditions that make it unsuitable for crop production. Increased production is now obtained mostly by using present cropland more intensively. New crop varieties and increased fertilization are important factors producing higher yields. More intensive cropping systems increase row crops and grain crops at the expense of forage crops. Multiple cropping has increased, and the rest period in the slash-and-burn system has been reduced or eliminated in many tropical areas. The effect of these changes on soil erosion has been mixed. Fertilization and multiple cropping increase plant cover on land and reduce erosion. The replacement of forage crops with row crops and grain crops and the shortening of rest periods in slash-and-burn tend to increase erosion.

12 EROSION PROBLEMS Erosion occurs in many forms as a result of several causes. Anything that moves, including water, wind, glaciers, animals, and vehicles, can be erosive. Gravity pulls soil downslope either very slowly as in soil creep or very rapidly as in landslides. 12.1 Intermittent Erosion Erosion can be uniform and subtle. Sheet erosion, for example, removes layer after layer a little at a time until a lot of soil has escaped almost undetected. Most erosion, though, is intermittent and spotty. Surface irregularities concentrate the erosive effect of either wind or water in certain spots. Cavities may be blown out by wind or gullies cut by water. The pattern is usually spotty, as illustrated in Figure 12. Generally, more than half of the annual soil loss in an area occurs in only a few storms during which rain and wind are intense and plant cover is at a minimum. Weeks, months, or even years may pass without much soil being lost. The loss from a single ferocious storm sometimes exceeds that of an entire century. The spotty and intermittent nature of erosion complicates the interpretation of erosion measurements. A field with an average soil loss of 4 tons/ac (9 mt/ha) annually is within the accepted tolerable rate for most deep soils if the loss is evenly distributed. But if most of the loss comes from part of the field eroding at 40 tons/ac, that part of the field is being ruined by erosion. Furthermore, crops on adjoining areas may be suffering damage from sedimentation, as shown in Figure 13. An average over time is equally deceptive. The benefits of having only small soil losses for nine years are wiped out if severe loss during the tenth year completely destroys a crop and carries away the topsoil that produced it. 12.2 Accelerated Erosion The normal rate of erosion under natural vegetation is in approximate equilibrium with the rate of soil formation. A particular set of conditions maintains sufficient soil depth to insulate the underlying parent material from weathering just enough so that soil is formed as

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Conserving Soil and Water

Chap. 1

LEGEND Drainage basin divide Sub-drainage basin divide Stream Diffuse stream within swamp Tons/ac-yr 0-0.5 0.5-1 1-10 10-50 >50 0 0.5 mile 1

Figure 12 Estimated annual soil-loss rates by 10-acre (4-ha) cells in the Lake Canadarago drainage basin, New York. (From Kling and Olson, 1975.)

fast as it is lost. Deviations from equilibrium cause the soil to get either thicker or thinner until a new equilibrium is established. Precise data on rates of geologic (natural) erosion and soil formation are difficult to obtain but are thought to average about 0.5 ton/ac (1 mt/ha) annually (see Section 38). Tilling cropland, grazing pasture or rangeland, or cutting trees nearly always increases the rate of soil erosion. Loss of soil cover reduces protection and may accelerate soil loss by a factor of 10, 20, 50, or 100 times. Formation of new soil cannot keep pace with greatly accelerated erosion rates, so the soil becomes progressively thinner, sometimes until little or no soil remains. The quality of the remaining soil generally deteriorates, not only because the soil has less depth but also because its physical, chemical, and biological properties become less favorable for plant growth (Lal et al., 1999). Islam and Weil (2000) suggest that microbial biomass, specific respiration rate, and aggregate stability are good indicators of soil quality.

Sec. 12

Erosion Problems

5

Figure 13 Sediment from the higher areas covered and killed the crop in the foreground in this Iowa field. (Courtesy USDA Natural Resources Conservation Service.)

Accelerated erosion reduces the amount of plant growth a soil is able to support. The productive potential is reduced even if the actual production is maintained or increased by the use of fertilizer and other management techniques. A shallower soil, with its reduced capacity for storing water and plant nutrients and its generally poorer structure and aeration, cannot match the productive potential of the uneroded soil. 12.3 An Old Problem in a New Setting Cultivated fields, overgrazed pastures, and cutover forestlands have suffered from erosion since the dawn of civilization in all parts of the world. The eroded soil becomes sediment that covers bottomlands and sometimes becomes so thick that it buries both fields and cities. The result becomes an archaeologists treasure when a famous city such as Babylon is uncovered centuries after its inhabitants lost a frustrating battle with sediment eroded from nearby hills. Gullies, sand dunes, and other obvious signs of erosion have caused concern since the beginning of agriculture. Impressive terrace systems were built thousands of years ago to stop erosion. Even so, entire soil profiles have been lost by sheet erosion, gullies have dissected hillsides, and sand dunes have drifted across anything in their path, such as the sidewalk shown in Figure 14. Many millions of acres of formerly productive land have been abandoned because of erosion damage. In recent years, a new concern has been added to the age-old problems of erosion and deposition. Dust clouds and muddy water signify air and water pollution. Soil particles carry plant nutrients and other chemicals that contaminate water. Erosion has become an environmental problem that must be remedied for the sake of clean air and water. This new

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Conserving Soil and Water

Chap. 1

Figure 14 Sand from a nearby beach drifted into the city and blocked this sidewalk in Montevideo, Uruguay. (Courtesy F. R. Troeh.)

concern has added an urgency to erosion control that should have been recognized earlier. Similarly, the increasing amounts of waste materials such as sewage sludge and mine tailings that are being spread on soil as a means of disposal generate concerns of soil pollution, especially by accumulation of heavy metals in the soil (Dinel et al., 2000). Soil and water pollution concerns are addressed in Chapters 16 and 17. An increasing part of the impetus for soil conservation, especially that which is legally mandated, stems from environmental concerns. The early stages of pollution control concentrated on point sources such as sewage systems and smokestacks. Current efforts are beginning to include nonpoint sources such as soil erosion. Soil conservation practices must be used along with other pollution controls to protect the environment. 12.4 A Concern for All People Eroded soil and the chemicals it carries are matters of concern because a degraded environment harms everyones health and enjoyment. Polluted water, for example, is unsafe for drinking, swimming, and many other uses. It can kill fish; moreover, the surviving fish may impair the health and reproductive capacity of birds that eat them. Both the fish and the birds may be made unfit for human food. Erosion adds to the cost of producing food and other soil products, thereby increasing the cost of living. With worldwide trade and emergency relief programs, the effects of reduced production in any major area spread through the world markets. Eswaran et al., (2001) point out that soil erosion, soil compaction, and plant nutrient depletion are worldwide problems that add billions of dollars per year to the cost of food production. In extreme conditions, ruined land must be taken out of production, and the increased load placed on the remaining land drives up production costs. Installing expensive erosion control practices also adds to production costs, but these practices help assure that production will continue.

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Obstacles to Conservation

7

Soil conservation legislation should be of concern to all voters, even those not directly affected by it. Government may provide too little, too much, or even the wrong kind of control in an effort to bring about effective soil conservation. Tax funds are used to pay the publics share of conservation costs. The public needs to understand and support the principles of soil and water conservation and environmental protection.

13 OBSTACLES TO CONSERVATION Conservation is difficult to oppose, yet easy to overlook or ignore. Too many people give lip service to conservation but leave the application to someone else. Reasons for inaction include economic and aesthetic obstacles, insecurity and uncertainty, ignorance, and apathy. 13.1 Economic Obstacles Major decisions are usually based largely on economic considerations. How much will it cost? What returns can be expected? Will the cost be repaid in a short time, in a long time, or not at all? Much reluctance to apply conservation practices is based on economics. The people who must spend money to conserve their soil are not the only ones who suffer if the soil is eroded or benefit if it is conserved. Often the persons most affected live someplace downstream or downwind or will live at a later time. People are commonly reluctant to spend their money for unknown beneficiaries; some are unwilling to spend money to conserve soil for their own future benefit. Conservation practices vary greatly in costs, returns, and effectiveness. The easiest practices to promote are those like a good fertilizer program that will both conserve soil and return a profit within a short time. Longer-term practices such as liming and soil drainage may be recognized as desirable for some time before any action is taken. The time lag is still longer for terracing and other practices whose high investment costs require many years to repay. Least popular of all are practices such as changing to a less intensive land use with lower probable returns. The economic value of many conservation practices is further complicated by benefits that accrue to persons other than those who install the practices. Reduced erosion generally means there will be less air and water pollution and probably less flood damage in downstream areas. Consideration of externalities shows that many conservation practices are economically desirable for society as a whole even though their costs exceed the on-farm benefits (Stonehouse and Protz, 1993). The farmer should not be the only one involved in the decision nor the only one involved in paying for such practices. This kind of situation may be resolved by governmental involvement in the form of laws and cost sharing for conservation practices. 13.2 Aesthetic and Cultural Obstacles A great deal of pride can be involved in certain agricultural traditions. Straight rows, for example, are considered a mark of skill. Years ago, young farm workers were instructed Dont look back! because a tug on the reins would turn the horses and make a crooked row. Straight rows are appealing, but they cause erosion on hilly land by providing channels for runoff water to erode. Contour tillage is often the solution, but it must overcome tradition.

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Conserving Soil and Water

Chap. 1

Figure 15 Much hand labor is used in areas where people are barely able to subsist by tilling the land. (Courtesy F. Botts, Food and Agriculture Organization of the United Nations.)

Many farmers take pride in plowing so that crop residues are completely covered. Unfortunately, this practice exposes the soil to the impact of rain, runoff, and wind. Conservation tillage reduces erosion by leaving residues on the surface. This concept is now widely recognized, but it still must overcome tradition before many of its critics will accept it. 13.3 Insecurity, Uncertainty, and Small Holdings Many people in developing countries can barely eke out a living from their land by hard work such as the hand tillage shown in Figure 15. They know that traditional management has kept them and their predecessors alive, and that they have nothing to spare for gambling on a new method. It is difficult for them to change their techniques even for immediate benefits such as higher yields and less soil loss. It is still more difficult for them to adopt a practice that requires an investment, especially if the benefits are delayed or distributed over several years. The establishment of conservation practices under such conditions requires a reliable guarantee that these people will not starve to death if the new practice fails (Napier and Sommers, 1993). Short-term tenancy prevents the adoption of many desirable practices. A one-year contract, or even a five-year contract, does not give the renter enough time to benefit from the sizable investment of money and labor required to install long-term conservation practices. Theoretically, the landowners should be willing to invest in sound long-term practices, but many owners are too far removed from the land to realize what practices are needed. Short-term tenancy makes it easy for both tenants and owners to overlook problems, even when those problems reach critical stages.

Sec. 14

Conservation Viewpoint

9

Small holdings are a common problem in developing countries. They may need conservation structures that cross several property lines or even need to be applied to an entire watershed to be effective (Pandey, 2001). Often, much of the benefit would go to people living downstream in the form of flood control and pollution prevention. Individuals cannot be expected to apply such practices. 13.4 Ignorance and Apathy Most erosion occurs so gradually and subtly that its effects are easily overlooked until long after preventive action should have been taken. Even rills (small erosion channels) in a field are often ignored because tillage operations can smooth the surface again. Unproductive subsoil exposed on the shoulder of a hill is overlooked if the rest of the field remains productive. Even people who work with land often are unaware of how many tons of soil are being lost each year, of how costly these losses are, and of how short the useful life expectancy may be for a rapidly eroding soil. Many people are apathetic about future needs and have short-term viewpoints regarding the use of soil and other resources. Land that was ruined in the past is unavailable now, and land that is ruined now is lost to future generations. Reduced productivity of eroded but usable land is even more important because it is more widespread. Erosion-control practices needed to prevent environmental pollution often are not installed or are long postponed because of indifference. Some landowners claim the right to use their land as they please even if it is being ruined and even if the sediment is damaging other peoples property. Public opinion and environmental considerations have provided the impetus for laws restricting the rate of soil erosion allowable under certain conditions.

14 CONSERVATION VIEWPOINT The need for soil conservation has been clear enough to catch the attention of both modern and ancient people. For example, the people of ancient Rome, India, Peru, and several other places valued soil enough to build terraces that still stand today, such as those shown in Figure 16. Terrace walls were built of stones left on eroded hillsides; then laborers carried soil in baskets on their backs or heads from the foot of the hill up to the terraces to make level benches. The Chinese still carry out similar laborious projects, but most modern conservation structures are built with the aid of machines. Concern for the land is the most important characteristic of a soil conservationist. Those who have such concern will find a way to conserve their soil and water; those who lack concern often neglect to use even the most obvious and inexpensive means of conservation. Conservation efforts, therefore, include education and persuasion aimed at convincing more people to care for their land. Several organized groups now exist to promote soil and water conservation. The Natural Resources Conservation Service of the U.S. Department of Agriculture helps people install conservation practices; several other agencies assist their efforts. Employees of the Natural Resources Conservation Service work in cooperation with local Soil Conservation Districts that have their own national association. Interested individuals can become members of the Soil and Water Conservation Society, and there are many other groups at national, state, and local levels that advocate conservation of natural resources.

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Chap. 1

Figure 16 These terraces in Bolivia were built hundreds, or perhaps thousands, of years ago and are still protecting the soil from erosion. (Courtesy F. R. Troeh.)

Conservationists take a long-term view regarding the use of resources. Some land has been used for several thousand years and is still productive. All land needs to be used in ways that will maintain its usefulness. The objective of soil conservation has been stated as the use of each acre of agricultural land within its capabilities and the treatment of each acre of agricultural land in accordance with its needs for protection and improvement.

15 CONSERVATION TECHNIQUES The practices used for conserving soil and water are many and varied. Some practices are expensive and some only require new habits; some are permanent and some are temporary; some are limited to very specific conditions whereas others are widely useful, although none have universal application. The amount of soil and water saved varies from one practice to another and from one set of circumstances to another. 15.1 Land Use and Management One of the first items a soil conservationist considers is the use of land within its capabilities. Some land is suited for intensive cropping, especially where the soil is deep, level, fertile, well drained, and has favorable texture and structure. Other land is so steep, shallow, stony, or otherwise limited that it is suitable only for wildlife or other nondisruptive uses. Most land is suitable for some uses but unsuitable for others. Land use can be broadly classified into cropland, pastureland, woodland, wildlife and recreational land, and miscellaneous use. Each broad class can be subdivided several

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Figure 17 A dense growth of bluegrass in this lawn provides excellent protection against erosion. (Courtesy F. R. Troeh.)

times. For example, cropland may be used for cultivated row crops, small grain crops, or hay crops. The soil exposure to erosive forces declines from cropland to pasture and woodland and then to wildlife land. These latter uses are therefore considered to be progressively less intense. Nonagricultural classes may parallel agricultural ones. For example, lawn grasses, as shown in Figure 17, might be roughly equivalent to a similar growth of pasture grasses. Management can alter the erosive effects of land use. Row crops, for example, can be grown in wide or narrow rows that may or may not follow contour lines. The time of exposure to the elements between the harvesting of one crop and the protective growth of the next varies considerably. The soil may or may not be protected by crop residues or by special cover crops during periods when the main crop is not on the land. These variables have considerable effect on the amount of erosion that is likely to occur. Variations also occur with other types of land use. Pasture, for example, may have grasses and legumes that were selected to provide good ground cover and forage, or it may have whatever happens to grow. The number of livestock may be limited to what the pasture can readily support, or overgrazing may kill much of the vegetation. Extreme over- and under-use may occur in the same pasture if the animals spend too much time in one area. Also, both soil and vegetation may be damaged by trampling if livestock are allowed to graze when the soil is too wet. 15.2 Vegetative and Mechanical Practices Conservation techniques are often divided into vegetative and mechanical practices. There is no good reason for always favoring one type over the other; both include a wide variety

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of methods for protecting soil against erosive forces. Often, the best approach is to use a combination of vegetative and mechanical practices. Vegetative practices include techniques that provide denser vegetative cover for a larger percentage of the time. Changing to less intensive land use usually reduces erosion considerably. The problem is that less intensive land use is usually less profitable. A crop rotation provides a compromise by using a series of different crops, some providing more income and some giving more soil protection. Crops grown for the purpose of protecting soil between other crops are known as cover crops. Choices of land use, crop rotations, and cover crops need to be accompanied by good management practices that help each crop grow well. Good seed planted at the right time in a proper seedbed helps get the crop off to a good start. Adequate fertilizer and lime where needed promote vigorous growth. Narrow row spacing allows a row crop to provide better soil cover sooner. These management techniques generally improve both yield and erosion control. Special vegetation is needed in critical places. Grassed waterways can prevent the formation of gullies. Windbreaks can direct air currents away from erodible land. Various forms of strip cropping reduce water erosion, wind erosion, and pollution. Appropriate plantings in odd corners, steep slopes, or other problem areas provide food and cover for wildlife as well as erosion control. Disturbed areas such as roadbanks and mine spoils need special plantings. Vegetation can limit erosion to geologic rates (the rate of erosion under native vegetation defines the geologic rate for a particular setting). Grasses, trees, and other plants are natures tools for controlling erosion. Although geologic rates are usually quite slow, they occasionally are as sudden and rapid as a landslide. Sometimes the rate of erosion should be reduced below the geologic rate by providing more than the natural amount of protection. More often, some increase above the geologic rate is permissible. Mechanical methods broaden the choice of vegetation and allow higher-income crops to be grown even though the crops provide less soil protection. Contour tillage, for example, often reduces erosion to half of that resulting from straight-line tillage. Tillage systems that leave more crop residues on the soil surface reduce erosion markedly. The ultimate in reduced tillage, a no-till system, is an excellent means of conserving soil. Its use is expanding rapidly, partly because modern herbicides are helping to make it practical to reduce or eliminate tillage. Additional erosion control can be achieved by building terrace systems, such as those shown in Figure 18, to hold soil in the field. Soil movement may occur between terraces, but the soil caught in terrace channels will not pollute a stream. Of course, the channels must be cleaned periodically as a part of terrace maintenance. Various structures made of concrete, wood, metal, or other sturdy material limit erosion by controlling water flow. Critical points occur where water must drop to a lower elevation. The water may be conducted through a pipeline, down a flume or chute, or over a drop structure. Pilings, riprap, or other bank protection may be used to keep a stream from meandering to a new location. Mechanical methods of erosion control tend to be either very inexpensive or very expensive. Conservation tillage saves fuel, time, and money by reducing the number of trips and the total amount of work done on the soil. Contour tillage may require more planning and layout, and it generally adds some inconvenience in the form of short rows, but the fuel requirement for working across the slope is usually slightly less than that for up and down the slope.

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Figure 18 Terraces such as these hold the soil on the field rather than letting it erode away. (Courtesy USDA Natural Resources Conservation Service.)

Tillage changes require no new investments unless new equipment is needed. However, these inexpensive practices are short-lived and must be repeated for each new crop. Most long-lasting mechanical methods of erosion control involve expensive structures such as terraces, dams, and drop structures. The earthmoving and concrete work required are costly. Expensive structures are usually justified by many years of usefulness and increased flexibility of land use. 15.3 Conserving Soil and Water Together Soil and water conservation are so interrelated that they must be accomplished together. There are very few techniques that conserve one but not the other. Both soil and water can be conserved by protecting the soil from raindrops that would puddle on the surface and produce a crust. Plant material intercepting raindrops helps maintain permeability so that water can infiltrate instead of running off. The soil acts as a reservoir that conserves water. Reducing both splash and runoff conserves soil. Contouring, contour strip cropping, rough surfaces created by tillage, and terracing all increase infiltration by holding water on the land. Any runoff that occurs is slower and carries less soil. Streams fed by seepage and slow runoff have more uniform flow and lower flood peaks than would occur from unprotected watersheds. Reducing erosion reduces the rate at which streams, ponds, and lakes fill with sediment. Reservoir capacities are thus maintained for recreation, flood control, power generation, and irrigation. Keeping sediment out of the water also lowers the supply of plant

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nutrients in the water and thereby reduces unwanted growth of algae and other vegetation. Soil particles absorb pollutants that are best kept out of water by keeping the soil on the land. The control of nonpoint sources of water pollution therefore centers on conserving soil. Wind erosion control is more closely related to water conservation than it might seem. Water conservation is very important for plant growth in dry climates. Anything that slows runoff and helps get more water into the soil or keeps it there by reducing evaporation provides more water for plant growth. Improved plant growth in turn helps reduce wind erosion as well as water erosion.

16 CHOOSING CONSERVATION PRACTICES Soil and water conservation is too complex to be solved by only one approach. Each situation needs to be analyzed to determine what problems and potentials exist and what alternatives are available. 16.1 Soil Properties That Influence Conservation Many soil properties influence soil and water conservation, but a few deserve special emphasis because they strongly influence runoff and erosion control. Soil topography, depth, permeability, texture, structure, and fertility are worth consideration in relation to conservation. Topography includes the gradient, length, shape, and aspect (direction) of slopes. These features control the concentration or dispersion of erosive forces such as runoff water and wind. Topography also influences the practicality of erosion control practices such as contouring, strip cropping, and terracing. These practices may be very helpful on long, smooth slopes but impractical on rolling topography with short, variable slopes. Soil depth, the nature and thickness of soil horizons, and the type of underlying rock material all affect the rate of soil formation. The tolerable rate of erosion is much lower for shallow soils over hard bedrock than for deep soils underlain by loess or other unconsolidated material. Subsoils with high clay contents or other unfavorable properties need a covering of topsoil to support plant growth. Deep soil is favorable for water storage and plant growth. Where the soil is shallow, it may be impossible to smooth the land for drainage and irrigation or to move soil to build terraces and ponds. Soil permeability helps determine how much water will run off and cause erosion. Soil permeability is most commonly limited by a soil surface puddled by raindrops or traffic, plowsoles or other compact layers, heavy subsoils with small water passages, frozen soil, and bedrock or cemented layers. Restrictive layers near the soil surface require little water to saturate the overlying soil and cause runoff to begin. Soil permeability also influences the functioning of subsurface drainage systems and septic tank drain fields. Soil texture and structure influence soil permeability and erodibility. Clay can bind soil either into a solid mass or into structural units with pore space between them. Individual clay particles are difficult to detach from soil but can be moved long distances after they are detached. Sand particles are easily detached from sandy soils, but fast-moving water is

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Choosing Conservation Practices

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Figure 19 Soil survey reports such as these contain soil maps, descriptions, and interpretations for various uses such as soil and water conservation. (Courtesy F. R. Troeh.)

required to transport them. Silty soils are often the most erodible by water, because the silt particles are too large to stick together well and are small enough to be transported readily. Silt particles are small enough, however, to resist detachment by wind unless they are knocked loose by something else, such as moving sand particles. Soil fertility is important to soil conservation because plant cover helps protect the soil. Vigorous growth produced on a fertile soil provides more complete cover and better protection than sparser growth. Fertilizer and lime are therefore important for soil conservation. 16.2 Maps for Conservation Planning Most of the soil properties discussed in the preceding section can be mapped. Topographic shapes and elevations are shown by contour lines. Soil depth, texture, structure, and many other properties are considered in naming the soil series shown on soil maps. Slope gradient and past erosion are also generally indicated on soil maps. Conservation plans are based on soil maps. The soil map units are classified on the basis of the intensity of land use for which they are suited and the treatment they need. Soil maps are often colored to make important features stand out for planning. The soil maps are published along with descriptions of the soils and interpretations for various uses in soil survey reports such as those shown in Figure 19. 16.3 Considering Alternatives Any piece of land could be used and managed in a variety of ways. Some ways would cause disastrous damage or monetary loss, but several satisfactory ways usually remain after

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unsuitable uses are eliminated. For example, a field might be used for pasture or hay production without any special practices, for a crop rotation without excessive erosion if contour strip cropping and conservation tillage were used, or for intensive row crops if terraces were built and conservation tillage used. Increased intensity of use normally requires additional conservation practices to protect the land. Economic factors and personal preference are usually considered when a choice must be made from alternatives such as these. Many choices depend on the type of agriculture being practiced. Growing hay or pasture on part or on all of ones land implies that the forage will be fed to livestock. Building terraces to permit more row crops fits a cash-crop operation. Cover crops can be used to protect the soil between the trees in an orchard. Irrigation makes it possible to grow a wide variety of crops in arid climates, and soil drainage permits previously wet areas to be cropped. 17 CARING FOR THE LAND Conservationists see the possession and use of land as a stewardship. The land that one person has now was previously someone elses and will soon pass to others. Its condition should be as good when passed on as when it was received. The owner has a responsibility to society for the way the land is used and the care it receives. The authority of governmental units to tax land, to place restrictions on its use, and to require that access and some other rights be granted to others indicates that ownership is not absolute. Soil and water conservation attitudes and practices are needed everywhere. Even the best land is subject to damage if it is abused. Good land, fair land, and poor land are all useful if they receive proper care. People need constant reminders not to choose short-term exploitation over long-term productivity. The use and care of agricultural land are stressed throughout this book, but the conservation needs of nonagricultural land must not be overlooked. Erosion on a construction site is often more rapid than in any nearby field. Excess traffic, especially by off-road use of motorcycles, four-wheel drive, all-terrain, or other vehicles, can start a gully. Modified versions of agricultural practices may control erosion in these and many other circumstances. Vegetation and mechanical structures can be adapted to a wide variety of situations. Large areas of land are devoted to raising grass for livestock or trees for wood products, recreation, and wildlife use. These uses can provide excellent protection for the land, but they can also be abused. Overgrazing a pasture degrades the vegetation and exposes the soil to erosive forces (Herrick et al., 1999). Similarly, road building, logging operations, or fire can open a forested area to landslides and gullying (Elliot et al., 1999). Population growth makes good land stewardship more crucial. How many people can the Earth support? is a pertinent question (Brown and Kane, 1995). Increasing pressure from higher population densities makes the conservation task both more important and more difficult. The need for population control has become obvious enough to cause many programs to be developed for that purpose, such as the family planning center shown in Figure 110. These programs and soil conservation practices are both needed, literally, for the salvation of the world.

Chap. 1

Summary

17

Figure 110 Family planning centers such as this one on Mauritius, an island in the Indian Ocean, are helping reduce birth rates and control population. This island has 850,000 people living in an area of 720 mi2 (1865 km2). (Courtesy P. Morin, Food and Agriculture Organization of the United Nations.)

SUMMARY Erosion of the soil resource often goes unnoticed. The loss in productive capacity is usually worse than the tonnage indicates because erosion sorts the soil and removes the most fertile parts. Soil conservation seeks ways to use the soil without losing it. Production increases formerly came mostly by cultivating more land. Now, increased production must be obtained by increasing yields and intensity of land use because new land is hard to find. Erosion is so intermittent and spotty that averages fail to reveal much of the serious damage done to soil and crops. Cultivated fields, overgrazed pastures, and cutover forest lands have suffered from erosion since the dawn of civilization, and sediment has been polluting streams and burying fields and cities. Erosion control efforts such as terrace systems have been in use for thousands of years but have been inadequate to prevent the loss of millions of acres of land. The contribution of erosion to air and water pollution has become a major concern in recent years. Erosion, pollution, and soil conservation are costly to everyone. The installation of conservation practices is costly, and people are reluctant to abandon traditional methods. Subsistence agriculture, short-term tenancy, ignorance of erosion

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problems, and apathy are additional obstacles, but people who care about the land find ways to conserve their soil and water. Many different techniques are available for conserving soil and water. The first requirement is to select an appropriate use within the land capability. Good management and conservation practices come next. Protective practices may be vegetative, mechanical, or a combination of the two. The effectiveness of vegetative practices depends on the density of the vegetation and the percentage of time it covers the land. Permanent vegetation, such as grassed waterways, windbreaks, or other plantings, can provide protection for vulnerable sites. Mechanical methods such as conservation tillage and water-control structures permit the growth of higher-income crops. Soil and water conservation must be accomplished together. Contouring, terracing, and protecting the soil surface against crusting all increase infiltration and conserve both soil and water. Keeping sediment out of water reduces pollution and lengthens the life of reservoirs. Water conservation in dry climates increases plant growth and reduces wind erosion. Soil properties such as topography, depth, permeability, texture, structure, and fertility influence the erodibility of soil and the best choice of conservation practices. Topographic maps and soil maps identify many of these properties and are useful for conservation planning. Good stewardship of land requires passing it on to others in good condition for continued productivity. Both agricultural and nonagricultural lands need soil and water conservation. Population growth makes land stewardship increasingly important. QUESTIONS1. 2. 3. 4. 5. 6. 7. 8. In what ways can average rates of erosion be misinterpreted? Why should a factory worker living in an apartment be concerned about erosion? Why do people fail to adopt new methods of erosion control? Why would one build expensive terraces to control runoff and erosion that could be controlled by inexpensive vegetative methods? What influence has increased environmental concern had on soil conservation? Why are different techniques needed to conserve soils of sandy, silty, and clayey textures? What information useful for conservation planning can be shown on maps? Why do some people do a much better job of soil and water conservation than others?

REFERENCESASHBY, J. A., J. A. BELTRN, M. DEL PILAR GUERRERO, and H. F. RAMOS, 1996. Improving the acceptability to farmers of soil conservation practices. J. Soil Water Cons. 51:309312. BROWN, L. R., and H. KANE, 1995. Reassessing population policy. J. Soil Water Cons. 50:150152. DEEVEY, E. S., JR., 1960. The human population. Sci. Am. 203(3):195204. DINEL, H., T. PAR, M. SCHNITZER, and N. PELZER, 2000. Direct land application of cement kiln dustand lime-sanitized biosolids: Extractability of trace metals and organic matter quality. Geoderma 96:307320. ELLIOT, W. J., D. PAGE-DUMROESE, and P. R. ROBICHAUD, 1999. The effects of forest management on erosion and soil productivity. Ch. 12 in R. Lal (ed.), Soil Quality and Soil Erosion. CRC Press, Boca Raton, FL, 329 p.

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References

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ESWARAN, H., R. LAL, and P. F. REICH, 2001. Land degradation: An overview. In E. M. Bridges, I. D. Hannam, L. R. Oldeman, F. W. T. Penning de Vries, S. J. Scherr, and S. Sombatpanit (eds.), Response to Land Degradation. Science Pub., Inc., Enfield, NH, 510 p. GALL, G. A. E., and G. H. ORIANS, 1992. Agriculture and biological conservation. Agric. Ecosystems Environ. 42:18. GREENLAND, D. J., 1994. Soil science and sustainable land management. Ch. 1 in J. K. Syers and D. L. Rimmer (eds.), Soil Science and Sustainable Land Management in the Tropics. Cab International, Wallingford, U.K., 290 p. HERRICK, J. E., M. A. WELTZ, J. D. REEDER, G. E. SCHUMAN, and J. R. SIMANTON, 1999. Rangeland soil erosion and soil quality: Role of soil resistance, resilience, and disturbance regime. Ch. 13 in R. Lal (ed.), Soil Quality and Soil Erosion. CRC Press, Boca Raton, FL, 329 p. ISLAM, K. R., and R. R. WEIL, 2000. Soil quality indicator properties in mid-Atlantic soils as influenced by conservation management. J. Soil Water Cons. 55:6978. KLING, G. F., and G. W. OLSON, 1975. Role of Computers in Land Use Planning. Information Bull. 88, Cornell Univ., Ithaca, NY, 12 p. LAL, R., 1999. Soil quality and food security: The global perspective. Ch. 1 in R. Lal (ed.), Soil Quality and Soil Erosion. CRC Press, Boca Raton, FL, 329 p. LAL, R., D. MOKMA, and B. LOWERY, 1999. Relation between soil quality and erosion. Ch. 14 in R. Lal (ed.), Soil Quality and Soil Erosion. CRC Press, Boca Raton, FL, 329 p. LEE, L. K., 1996. Sustainability and land-use dynamics. J. Soil Water Cons. 51:295. NAPIER, T. L., and D. G. SOMMERS, 1993. Soil conservation in the tropics: A prerequisite for societal development. In E. Baum, P. Wollf, and M. A. Zbisch (eds.), Acceptance of Soil and Water Conservation: Strategies and Technologies. DITSL, Witzenhausen, Germany, 458 p. NAPIER, T. L., S. M. NAPIER, and J. TVRDON, 2000. Soil and water conservation policies and programs: Successes and failures: A synthesis. Ch. 38 in T. L. Napier, S. M. Napier, and J. Tvrdon (eds.), Soil and Water Conservation Policies and Programs: Successes and Failures. CRC Press, Boca Raton, FL, 640 p. NOWAK, P. J., 1988. The costs of excessive soil erosion. J. Soil Water Cons. 43:307310. PANDEY, S., 2001. Adoption of soil conservation practices in developing countries: Policy and institutional factors. In E. M. Bridges, I. D. Hannam, L. R. Oldeman, F. W. T. Penning de Vries, S. J. Scherr, and S. Sombatpanit (eds.), Response to Land Degradation. Science Pub., Inc., Enfield, NH, 510 p. RYABCHIKOV, A. M., 1976. Problems of the environment in a global aspect. Geoforum 7:107113. STONEHOUSE, D. P., and R. PROTZ, 1993. Socio-economic perspectives on making conservation practices acceptable. In E. Baum, P. Wollf, and M. A. Zbisch (eds.), Acceptance of Soil and Water Conservation: Strategies and Technologies. DITSL, Witzenhausen, Germany, 458 p. TWEETEN L., 1995. The structure of agriculture: Implications for soil and water conservation. J. Soil Water Cons. 50:347351. WHLKE, W., G. HENGYUE, and A. NANSHAN, 1988. Agriculture, soil erosion, and fluvial processes in the basin of the Jialing Jiang (Sichuan Province, China). Geojournal 17:103115.

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2SOIL EROSION AND CIVILIZATIONThe earliest cultivated fields were small, with tillage likely restricted to more level, productive soils, and with short periods of cultivation followed by long periods of fallow (shifting cultivation). Erosion losses were not serious until populations multiplied, permanent settlements were established, and the cultivated acreage was expanded to meet the increased need for food. Sharpened sticks and stones were no longer adequate tools for clearing land and raising crops (Warkentin, 1999), so hoes and wooden plows were invented (see Chapter 9). More intensive and widespread cropping increased erosion, decreased soil productivity, destroyed considerable land, and added sediment to streams, lakes, and reservoirs.

21 ORIGIN OF AGRICULTURE Human beings were hunters and gatherers until relatively recent times. It is impossible to determine where crops were first cultivated, but relics from old village sites indicate the location and age of early tillage. Archaeologists in 1946 uncovered an ancient village at Jarmo in northern Iraq (Braidwood and Howe, 1960). One relic found in the dig was a stone hand sickle. Other stone implements found at the village site could have been used for tilling the soil and for weeding growing crops. This village was occupied about 11,000 B.C.E. and is considered to be the earliest known site of cultivated agriculture. Other prehistoric villages were found in the area dating from 11,000 to 9500 B.C.E. These villages were located on upland sites with friable, fertile, easily tilled, silt loam soils. The progenitors of modern domestic wheats and barleys are believed to have been among the grasses native to the area. Wild beans, lentils, and vetches were among the indigenous legumes. Rainfall probably amounted to 18 to 20 in. (450 to 500 mm) annually. Villages that were occupied during the period 9500 to 8800 B.C.E. were also excavated on lowland sites in the southern part of Iraq, near the Tigris and Euphrates rivers. The climate of this area is considerably drier than that farther north. Crop production by dryland farming methods was nearly impossible, so irrigation had to be invented and used. The20

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Erosion in the Cradle of Civilization

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record is clear that water from the rivers was used to produce crops. An abundant food supply enabled large cities, such as Babylon, to develop.

22 EROSION IN THE CRADLE OF CIVILIZATION Mesopotamia, an ancient country located between the Tigris and Euphrates rivers in what is now part of Iraq, is sometimes called the cradle of civilization. It is supposed to be the locale of the biblical Garden of Eden and the Tower of Babel. Lowdermilk (1953) recounted the early history of this area. The following is a condensed version of his account. Long ago, this area was covered by a great flood that wiped out the previous civilization and left a thick deposit of brown alluvium over the whole area. After the flood subsided, the area was resettled and known as Sumer. Kish was its capital city. Kish must have been a magnificent city in its day, but eventually its buildings were abandoned and buried by a thick layer of sand and silt eroded from the nearby hills. This sediment preserved the ruins until they were excavated by archaeologists in the early twentieth century. Babylon, one of the most famous cities of ancient times, succeeded Kish as the capital of Mesopotamia. King Nebuchadnezzar was proud of having built the city of Babylon. He also boasted of building a canal and irrigation system and of cutting down huge cedar trees from Mount Lebanon to erect magnificent palaces and temples. He did not know that cutting down the forest and allowing sheep and goats to overgraze the hillsides would cause massive erosion. Nor did he realize that erosion would cause the downfall of Babylon by filling its irrigation canals with silt faster than slaves could clean it out. Babylon, too, was abandoned and its buildings were buried under about 4 m (13 ft) of erosional debris (Juo and Wilding, 2001). It, too, was later excavated by archaeologists. The Sumerians of Mesopotamia numbered about 25 million at the peak of their power and prestige. By the 1930s, Iraq, a major part of ancient Mesopotamia, had a population of about four million. What happened to cause the area to lose most of its population? Lowdermilk suggests that the primary reason was that it was weakened by the failure of its agriculture as the canals silted full and the soils became saline. Irrigated fields of the southern, lowland region were not damaged by soil removal, rather they were ruined by sediment from eroded land above them. Demand for food forced cultivation higher up the steep slopes in the watershed to the north. Sheep and goats overgrazed the hill pastures, and trees were felled indiscriminately for lumber and fuel. These practices denuded the watershed, causing severe erosion and erratic river flow. Large sediment loads, carried by the rivers in the sloping areas where flow was rapid, settled out in the more level areas, and salts from the irrigation water accumulated in the soils. Much sediment was deposited in the irrigation canals and ditches. In time, human labor was insufficient to cope with removal, so sections of irrigated land were abandoned. Drifting, windblown soil from the sparsely covered, abandoned lands filled the remaining irrigation structures. Eventually, the whole irrigated area had to be abandoned. Large urban populations could no longer be supported, and the area became a virtual desert. The area is very badly gullied now, and much of the original soil is gone. This was the first of many failed attempts to develop a permanent, productive, cropland agriculture.

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Chap. 2

23 EROSION IN MEDITERRANEAN LANDS Communities on the trade routes between Mesopotamia and Egypt developed systems of arable agriculture shortly after tillage was first used. It is probable that the knowledge of cultivation and irrigation was carried from Mesopotamia to these countries by traders and other travelers. Unfortunately, much of the detailed history and knowledge was lost when the library at Alexandria burned in the third century C.E. (Warkentin, 1999). It was the greatest library of its time, with over a half million manuscripts. 23.1 Soil Productivity in Egypt Egypt had a dry climate. Only the narrow Nile River floodplain could be cultivated. Natural flooding provided a type of irrigation with no canals. Little soil eroded because the floodplain was level. The Nile River rises in the mountains and tablelands of Ethiopia far to the south of the irrigated Egyptian floodplain. Extensive forest cutting and cultivation of steep slopes in the upper reaches of the watershed caused considerable erosion. The coarser sediments were deposited mostly in the Sudan where the river left the high country and entered the more level plain. Sediment carried into Egypt was fine textured and fertile. The annual deposit helped maintain soil productivity. The system developed in the lower Nile valley was the first successful attempt to develop a permanently productive, cultivated agriculture with irrigation. But the High Aswan Dam, completed on the Nile River 600 mi (1000 km) south of Cairo in 1970, has upset the precarious balance that kept the floodplain productive (Thoroux, 1997). The new dam regulates the flow of the river, reduces major flooding, and traps most of the revitalizing silt in Lake Nasser, behind the dam. It was the annual flood with its regular fertile silt deposit that was largely responsible for the maintenance of the deltas productivity. What will happen now? Egypt and the world are anxiously watching to see what the final answer will be. Bedouins have a long history of grazing livestock on the uplands above the valley of the Nile. The sparse vegetation in this desert area makes it subject to wind erosion, especially if overgrazed, and requires flocks to range over a large area. Some Bedouins have small areas of cropland in favorable sites. Briggs et al. (1998) discuss the reasons why some Bedouins choose to crop sites in the channel of a small stream (which is dry most of the time but stores water in the soil when there is runoff), in the floodplain adjacent to a major stream, or on the edge of Lake Nasser. The problem is that any of these sites is subject to flooding during wet periods and to drought at other times. 23.2 Erosion in Israel, Lebanon, Jordan, and Syria The ancient lands of Canaan, Phoenicia, and Syria have long had sedentary populations and established, cultivated agricultures. Cultivation was restricted initially to gently sloping lowland areas, with flocks and herds utilizing the steeper-sloping lands as range. The gradual encroachment of cultivation on the steeper lands and the reduction of protective native cover by overgrazing and timber harvesting increased runoff, erosion, and sedimentation. The Phoenicians were among the first people to experience severe erosion on steep cultivated slopes. They found that bench terraces made by constructing stone walls on the contour and leveling the soil above them reduced water erosion and made irrigation on

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steeply sloping land possible. The ancient terraces in this region are still being cultivated successfully. Large soil losses have occurred from nonterraced sloping land through the years. Extremely severe erosion has occurred on more than a million acres of rolling limestone soils between Antskye (Antioch), now in Turkey, and Allepo, in northern Syria. From 3 to 6 ft (1 to 2 m) of soil has been removed. Half of the upland soil area east of the Jordan River and around the Sea of Galilee has been eroded down to bedrock. Much of the eroded material was deposited in the valleys, where it is still being cultivated. Even these floodplain soils are subject to erosion, however, and gullies are cutting ever deeper into them. Archaeologists found the former city of Jerash buried on a floodplain under as much as 13 ft (4 m) of erosional debris. Some arid regions, such as the Sinai Peninsula, have been so severely overgrazed that the land is cut by extremely large gullies, despite low rainfall. Winds severely eroded the soil between the gullies, but as stones of various sizes were exposed, a closely fitted desert pavement formed that now prevents further wind damage. (See Figure 51.) The productive capacity of the soil has greatly deteriorated. As a result, population in many sections has been drastically reduced by starvation and emigration. Some areas that formerly produced food for export to the Roman and Greek empires now cannot produce enough to feed the small indigenous population. Some authorities blame the decline in productivity, particularly in the drier sections, on a change of climate, but agricultural scientists are convinced that the climate has not changed sufficiently to account for this decline. They believe that soil loss due to water and wind erosion is the root of the problem (Le Houerou, 1976). In spite of the damage done to the soils in this region, there is still hope for cultivation agriculture. Burgeoning population has forced the Israelis to produce as much food as possible locally. They are cultivating much more land than was ever tilled in their country previously, and with good results in spite of the dry climate. They are employing excellent farming practices, including the use of terraces and other soil and water conservation measures. 23.3 Erosion in Northern Africa and in Southern Europe Knowledge of cultivation spread westward from Egypt and the Middle East to northern Africa and to Greece, Italy, and other parts of Europe. Erosion in Northern Africa. Tunisia and Algeria, on the Mediterranean seacoast, with annual precipitation of about 40 in. (1000 mm), produced an abundance of crops in the early Roman era. There was substantial production even inland, where precipitation was much lower. Carthagineans, residents of present-day Tunisia, were excellent farmers; cultivation techniques were advanced for the times, and yields were good. They used very careful water conservation methods and had extensive irrigation works. Water-spreading techniques were employed in many of the drier regions. Relics of grain-storage structures and oliveoil presses attest to the farmers expertise and to the regions productivity. Grain produced in excess of local needs was exported. Serious deterioration took place over the centuries despite the high initial productivity of the soils. Winter rainfall caused erosion on bare soils, and wind erosion often destroyed soils left without cover during drier parts of the year. After the fall of Rome,

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Carthage was attacked by desert dwellers from the south (herders mostly), and the regions agriculture declined. Vegetative deterioration caused by neglect and overgrazing further reduced soil productivity. Much formerly productive land lost all of its topsoil as it was eroded down to a stony desert pavement. Desert encroached onto productive cultivated fields; the potential for food production was drastically reduced. Soils in many areas that once supported large populations now provide food for only a few hundred people. Lowdermilk (1953) mentioned the ruined city of El Jem on the plains of Tunisia whose amphitheater could accommodate 65,000 people. He wrote that in the late 1930s there were fewer than 5000 inhabitants in the whole district surrounding the citys ruins. El Jems cultivation agriculture was destroyed after only a short period of successful production. Local wars and invasion played a part in the massive decrease in production, but soil erosion was the main cause of the deterioration and ultimate demise of a productive system. Erosion in Greece and Italy. The Greeks originally were a pastoral people. The upland areas of their country were covered with forests; the productive lowland soils were used for grazing. Farming gradually replaced flocks and herds. Productive valley soils were cultivated first but, as population increased, food needs demanded production from land higher up the hillsides. Old Greek agricultural literature describes a few special soil-management practices, such as multiple cultivations, fallowing for one to several years, and deep plowing, which help to maintain productivity or at least reduce the rate of its decline. Soil erosion and soil deterioration, however, were rarely mentioned. Apparently they used no special erosioncontrol practices. More than 3 ft (1 m) of soil was washed from the surface of extensive areas, and in places the soil was eroded to bedrock. Severe gullying occurred on the steeper slopes, and lower-lying fields were buried under unproductive erosion debris. Increased food needs and declining production from their soils caused the Greeks to exploit the grain-producing potential of their colonies in Italy, in northern Africa, and on Crete and smaller islands in the Mediterranean. The cheap grain imported from the colonies caused Greek farmers to shift to the growth of more profitable olives, grapes, and vegetables. Some areas that had not been too seriously impoverished continued to produce food grain to meet the countrys emergency needs resulting from acts of war, piracy, and bad storms. Italy was a colony of Greece for several centuries before Rome established its own empire. Initially, little land was cultivated, but when tillage was introduced both dryland and irrigation farming methods were used. Italy eventually became a granary for Greece. When Rome first became independent of Greece, it produced all its food needs locally. Agriculture was a prominent and respected vocation. Many Roman authors assembled knowledge of successful farming methods used at home and abroad. Widespread use was made of fallow; the legume crop, alfalfa (lucerne), was highly recommended as a valuable forage and as a soil-fertility-improving crop. Other legumes were recommended and used also. As Roman population increased, demands for food also increased. The level bottomland areas were insufficient to produce what was needed, so cultivation moved farther up the hillsides. Erosion from the uplands accelerated and became even more serious as forests were felled to supply timber for ships and for fuel. Denuded soils and huge gullies resulted. Dedicated and industrious farmers terraced and contoured much of their land to reduce erosion. Figure 21 shows some terraces currently in use in northern Italy. Sediment still washed into streams and was deposited in irrigation works, making regular removal necessary. When the empire deteriorated as a result of wars and invasions,

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Figure 21 Numerous terraces help to control erosion on this cropland near Miniato in northern Italy. (Courtesy F. R. Troeh.)

labor for sediment removal became scarce, and irrigated fields were progressively abandoned. Sedimentation in the major river channels caused frequent and destructive floods. As swamps developed close to the streams, malaria and other diseases increased in severity, forcing people to move to higher ground and to intensify farming on the steeper slopes. This caused still larger soil losses. With growing populations, deteriorating soils, and smaller yields, Rome had to depend on imported grain, particularly from Carthage, Libya, and Egypt. Cheap imported grains and declining soil productivity forced farmers to abandon some fields and to switch to the production of olives, vegetables, and grapes on others, as shown in Figure 22. Italian soils, despite severe damage, were more durable than those of Greece, and recuperated after the fields were abandoned. Reasonable yields can be produced with modern technology on most soils, but erosion is a continuing threat.

24 EROSION IN EUROPE Western Europe, north of the major mountain ranges (Alps, Jura, and Pyrennes), has a forest climax vegetation. Rainfall is abundant in most areas. Local agriculture was generally improved when Roman methods were introduced. 24.1 Erosion in the United Kingdom The Celts, pre-Roman inhabitants of Britain, had a well-developed, arable agriculture. They cultivated fields across the slope. Gully erosion was not a serious problem because rainfall was gentle. Washed sediment gradually built up on the downhill side of each field as a result

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Figure 22 This vineyard near Florence, Italy, has widely spaced rows that allow room for soilconserving grain and hay crops. Hillside ditches provide drainage below each row of grapes. (Courtesy F. R. Troeh.)

of sheet erosion and of tillage-induced downhill movement (Bennett, 1939). The Celtic agricultural methods were altered but not replaced by Roman techniques. The Saxons, who later came to Britain from mainland Europe, introduced a system of long, narrow, cultivated fields, mainly on the more level lowlands. Most of the old Celtic fields were then abandoned, many permanently. Erosion has been recognized in Great Britain since the middle 1800s, but it has not been widespread, and annual losses generally are not large. The extent and magnitude of erosion has been studied on a national scale since the late 1970s. Most measured losses are in the 12 t/ac-yr (24 mt/ha-yr) range, although some fields have lost up to 20 t/ac-yr (45 mt/ha-yr) under unusual conditions. Most erosion is blamed on shifts from ley farming to straight cereal production, with little or no animal manure or green manure returned to the soil. Overgrazing by sheep has also been blamed for some instances of erosion. Soil erosion was more severe in Scotland than in England because more steeply sloping areas were cultivated, soils were sandier, and amounts of rainfall were greater there (Arden-Clarke and Evans, 1993). The detrimental effects of erosion were recognized early in Scotland, and conservation practices were developed. Contour ridges were recommended and used, and a predecessor of the graded terrace was developed. 24.2 Erosion in France, Germany, and Switzerland Erosion became extremely severe in many hilly areas of central Europe as steep slopes were cleared of forest cover and cultivated. More than a thousand years ago, the farmers in what is now France returned some cultivated land to forest and developed bench terraces. Some terraces were constructed on slopes as steep as 100% (45). The soils on the benched areas

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Figure 23 Steep areas in the Swiss Alps are either left in forest or grazed judiciously. (Courtesy F. R. Troeh.)

were turned deeply every 15 to 30 years as the soils became tired. These terraced areas are still being used. Lowdermilk (1953) suggested that the Phoenicians were responsible for these developments. Appropriate land use and careful husbandry reduced erosion losses over most of the region. The steep lands at high elevations are now used only for forest and pasture, as shown in Figure 23. Moderate slopes at lower elevations produce grain crops, as in Figure 24. Mainly truck crops are grown on level bottomlands, as shown in Figure 25. Wind erosion is a serious menace in Europe also. Sandy soils subject to wind damage are found along seacoasts and inland where water-laid or glaciated coarse deposits occur. Systems of revegetation have been developed to hold the sands and prevent drifting in most areas where wind erosion poses a threat. These sandy soils are generally used for pasture or forest and are rarely cultivated. 24.3 Soil Reclamation in the Low Countries The Low Countries include most of The Netherlands and a part of Belgium. They occur on a generally flat plain with little land more than 150 ft (50 m) above sea level. Instead of losing land by soil deterioration, arable area has been increased by reclaiming land from the sea. This is a very expensive method of acquiring land, but it is justified by the extreme population pressure of the area. The new land is obtained by building dikes such as the one shown in Figure 26; pumping the salt water out, originally with windmills such as the one in Figure 27, now with large electric pumps; and using river water for reclamation and irrigation (Note 21). This new land is a mixture of sandy soils, clayey soils, and peat. Water erosion was never serious on these lands, and wind erosion has been well controlled by protective vegetation on sandy sites. The soils in this area are generally more productive now than they were when first reclaimed.

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Figure 24 Upland areas in Switzerland are cropped where the soils are favorable and the slopes are not too steep. Pasture and woodland are interspersed with the cropland. (Courtesy F. R. Troeh.)

Figure 25 Level alluvial soils in Swiss valleys are used for vegetables, such as the lettuce in this field, and other high-value crops. (Courtesy F. R. Troeh.)

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Figure 26 Dike protecting lowlands in the southern part of The Netherlands. (Courtesy F. R. Troeh.)

Figure 27 Windmill near Kapelle, The Netherlands. Windmills were the original power source for pumping water to reclaim the polders. (Courtesy F. R. Troeh.)

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NOTE 21

RECLAMATION OF THE POLDERS The people of The Netherlands and Belgium have been reclaiming land from the sea for hundreds of years to help meet the needs of dense populations. Most of the reclaimed land was near sea level and much of it was reclaimed in relatively small tracts. The polders form a much larger and better-known reclamation project than any of the others. The general plans for large-scale reclamation were drafted about 1890 by Cornelis Lely, but initiation was delayed until the 1930s. The Zuiderzee, an arm of the North Sea that reached deep into the Netherlands, was cut off and converted into a body of freshwater called the Ijsselmeer. Five polders covering about 545,000 ac (220,000 ha) have been reclaimed within the Ijsselmeer by the following procedures: 1. The Afsluitijk, a barrier dam 20 mi (30 km) long and nearly 325 ft (100 m) wide, was completed in 1932 to form the Ijsselmeer. River inflow gradually converted it to a freshwater lake. Excess river water is emptied into the ocean through sluice gates. 2. An area within the Ijsselmeer was surrounded by an inner dike, and the water was pumped out to form a polder. 3. Rushes were planted in the freshly drained polder to control weeds, use up water, and help aerate the soil. 4. Trenches, ditches, and canals were dug for drainage and irrigation. The rushes were burned and a crop of rape was planted the first year and wheat the second year. Both rape and wheat tolerate the initial saline conditions and are good soil conditioners. 5. After three to five years, the land was dry enough to replace the trenches with drain tile. Farmsteads were built, and the units leased to selected farmers. The inland border around the basin that contains the polders is a hilly area. Much of the cropland there is covered with highly erodible loess deposits. Erosion has been a problem ever since they began to clear the forests decades ago, but it became much more serious a